Issue #16560 has been updated by naruse (Yui NARUSE).

Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: REQUIRED to 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: DONE

ruby_2_7 e74d2a42b274844ed020ed121ee2f11c626c5fec.

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Bug #16560: Proc autosplats first argument if called with one argument and empty keyword splat
https://bugs.ruby-lang.org/issues/16560#change-84653

* Author: Dan0042 (Daniel DeLorme)
* Status: Closed
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: DONE
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While working on understanding the vm_args.c code via refactoring, I found the following:

```ruby
b = proc{ |a,b=0| [a,b] }
h = {k:42}
b.call([1,2,3],**h) #=> [[1, 2, 3], {:k=>42}]
h = {} 
b.call([1,2,3],**h) #=> [1, 2]           (in 2.7)
                    #=> [[1, 2, 3], {}]  (in 2.6)
```

Since the result is different from 2.6 I think this is a bug, especially since the result in 2.7 is so different based on being an empty or non-empty splat.

In my refactoring branch I'm solving this by moving the `args_check_block_arg0` check before the `ignore_keyword_hash_p` check, but that doesn't look so easy in master.



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